AC/Delco is a great plug and is the OE plug. I would install the exact same plug that it came with.
What is the spark plug gap setting for a 89 cadillac deville?
Distributor cap, rotor, plug wires or spark plugs could do that. Time for a tune up.
Spark plug gap is .028
.45
Most of these were .045 but you should check with a AC spark plug application book.
.035
0.045
Under the spark plug wires in the cylinder head, on top of the engine.
gap it to 44
.035
.060" is what the parts stores say.
.035 inch